1. Field of the Invention
The present invention relates generally to optical amplification suitable for optical fiber communication using wavelength division multiplexed optical signals including a plurality of optical carriers having different wavelengths, and more particularly to a method and device for such optical amplification and a system having the device.
2. Description of the Related Art
In recent years, a manufacturing technique and using technique for a low-loss (e.g., 0.2 dB/km) optical fiber have been established, and an optical communication system using the optical fiber as a transmission line has been put to practical use. Further, to compensate for losses in the optical fiber and thereby allow long-haul transmission, the use of an optical amplifier for amplifying an optical signal has been proposed or put to practical use.
An optical amplifier known in the art includes an optical amplifying medium to which an optical signal to be amplified is supplied and means for pumping (exciting) the optical amplifying medium so that the optical amplifying medium provides a gain band including the wavelength of the optical signal. For example, an erbium doped fiber amplifier (EDFA) includes an erbium doped fiber (EDF) as the optical amplifying medium and a pump light source for supplying pump light having a predetermined wavelength to the EDF. By preliminarily setting the wavelength of the pump light within a 0.98 .mu.m band or a 1.48 .mu.m band, a gain band including a wavelength of 1.55 .mu.m can be obtained. Further, another type optical amplifier having a semiconductor chip as the optical amplifying medium is also known. In this case, the pumping is performed by injecting an electric current into the semiconductor chip.
As a technique for increasing a transmission capacity by a single optical fiber, wavelength division multiplexing (WDM) is known. In a system adopting WDM, a plurality of optical carriers having different wavelengths are used. The plural optical carriers are individually modulated to thereby obtain a plurality of optical signals, which are wavelength division multiplexed by an optical multiplexer to obtain WDM optical signals, which are output to an optical fiber transmission line. On the receiving side, the WDM optical signals received are separated into individual optical signals by an optical demultiplexer, and transmitted data is reproduced according to each optical signal. Accordingly, by applying WDM, the transmission capacity in a single optical fiber can be increased according to the number of WDM channels.
In the case of incorporating an optical amplifier into a system adopting WDM, a transmission distance is limited by the wavelength dependence of gain which is represented by a gain tilt or gain deviation. For example, in an EDFA, it is known that complex gain characteristics are produced in a signal band, and the gain characteristics vary with total input power and pump light power to the EDFA.
To suppress the wavelength dependence of gain, combining an optical amplifier and an optical filter has been proposed. Configurations of the combination and effects obtained therefrom are described in detail in Japanese Patent Laid-open Nos. 3-44206 , 3-196125, and 8-213676 and OAA'90, MD1, pp44-47.
In the case that the optical filter is placed on the upstream side (input side) of the optical amplifier in this combination, the noise figure becomes high (bad). In this case, the insertion loss by the optical filter can be compensated by adjusting the gain of the optical amplifier. Conversely, in the case that the optical filter is placed on the downstream side (output side) of the optical amplifier, the noise figure is not high, but the optical output power is lowered (reduced) by the insertion loss by the optical filter. Accordingly, it is necessary in the prior art to select either intention to high optical output power while allowing an increase in noise figure or intention to low noise while allowing a decrease in optical output power.
The wavelength dependence of gain in a band of 1540 to 1560 nm can be suppressed by suitably setting the power of pump light especially in an EDFA. In the case that the power of pump light is relatively low, a positive gain tilt such that the gain increases with an increase in wavelength is obtained, whereas in the case that the power of pump light is relatively high, a negative gain tilt such that the gain decreases with an increase in wavelength is obtained. Accordingly, the pump light power can be controlled so that the gain tilt becomes flat. However, the pump light power required to make the gain tilt flat is generally high, so that a high-power pump light source is required. A laser diode for obtaining high-power pump light is expensive. Further, in the case of using a plurality of laser diodes in combination to obtain high-power pump light, an optical circuit becomes complicated in configuration.